Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head “flies” over a surface of the disc. This transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back.
Over the years, storage density of media has tended to increase and the size of storage systems has tended to decrease. This trend has led to a need for greater precision, which has resulted in tighter tolerance for components used in disc drives. In turn, achieving tighter tolerances in components requires increased precision in metrology systems for characterizing and parameterizing those components. Measuring angles of objects is one aspect of metrology, and measuring angles of conical cavities is of interest for some disc drive designs.
Metrology systems may include systems that use technology requiring contact with a workpiece as well as systems that obtain metrology data without contacting a workpiece. It is often the case that non-contact systems can be more precise than contact systems, but can be more expensive. Contact based systems can damage workpieces. What is needed is a low-cost, accurate, and repeatable metrology system that may be used, for example, in metrology of disc drive components.
The following description is presented to enable a person of ordinary skill in the art to make and use various aspects of the invention. Descriptions of specific materials, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the inventions. For example, aspects and examples may be employed in various metrology applications, including metrology of components of motors used in disc storage drives. Metrology equipment employing aspects disclosed herein may be designed and may operate in a number of ways. The exemplary apparatuses and systems provided herein are for illustrating various aspects and are not intended to limit the range of metrology apparatuses and systems in which examples and aspects may be applied.
FIG. 1 illustrates a cross-section of a conventional disc drive motor portion. The portion includes a hub 10 supporting discs 12. In operation, the hub 10 rotates about a fixed shaft 14. The fixed shaft 14 includes an upper shaft bearing cone 16 and a lower shaft bearing cone 18. An outer surface 34 of upper shaft bearing cone 16 forms an upper hydrodynamic bearing region 20 with opposing upper conical bearing sleeve 28. An outer surface 32 of the lower shaft bearing cone 18 forms a lower hydrodynamic bearing region 24 with opposing lower conical bearing sleeve 30. For proper operation, there should be an engineered fit between each of the shaft bearing cones 16 and 18 and respectively opposing conical bearing sleeves 28 and 30.
An aspect of this engineered fit is the angle at which the conical bearing sleeves 28 and 30 taper. To continue increasing disc drive performance, the angle at which conical bearing sleeves 28 and 30 taper will likely have to be increasingly controlled, for example to within 0.01 degrees or better of an engineered specification. In turn, determining whether conical bearing sleeves 28 and 30 are within 0.01 degrees of specification requires an accurate and repeatable metrology device and method. Since another factor considered in disc drive production is cost, the metrology device should be low cost. Cost may include such factors the metrology device's cost, and alsowhether the metrology system damages a workpiece being measured, and the speed at which a measurement can be completed.
FIG. 2 illustrates aspects of a conceptual method for deriving an angle 2θ of a conical cavity 208 (shown in cross-section) that exists, for example, in a conical bearing sleeve. A first sphere 212 having a known (or determinable) diameter is inserted in the conical cavity 208. A first height 204 associated with positioning of the first sphere 212 is measured. This measurement is made with respect to a reference such as illustrated at 202. The first sphere 212 may then be removed from conical cavity 208. A second sphere 210 is then inserted into the conical cavity 208. A second height 206 associated with positioning of the second sphere 210 is measured; second height 206 is preferably a measurement made with respect to the reference 202. After obtaining the first height 204 and the second height 206, an angle equal to one half of angle 2θ may be calculated by application of the formula below. R1 is the radius of the first sphere 212. R2 is the radius of the second sphere 210. H1 is the first height 204. H2 is the second height 206.
  θ  =      a    ⁢                  ⁢                  sin        ⁡                  [                                                    R                1                            -                              R                2                                                                    (                                                      H                    2                                    -                                      H                    1                                                  )                            -                              (                                                      R                    1                                    -                                      R                    2                                                  )                                              ]                            -        1            
FIGS. 3A-C illustrate schematic aspects of an exemplary metrology system 300 implementing aspects of the above sleeve cone angle measurement strategy. In illustrating these aspects, reference is made to sources of inaccuracy and non-repeatability in components of the metrology system 300. These sources of inaccuracy and non-repeatability combine into a complete measure of the accuracy and repeatability of the metrology system, commonly referred to as Gauge Repeatability and Reproducibility (GRR). Such references are by way of example and not limitation; other metrology systems created and operating by aspects presented herein may have fewer or additional sources of inaccuracy and non-repeatability.
Generally, a lower GRR signifies a more stable metrology device than a higher GRR. GRR has two primary components, repeatability and reproducibility. Repeatability is the ability of the same gauge to provide a consistent measurement during a number of uses by the same operator, and reproducibility is the ability of a gauge to give a consistent measurement regardless of the operator. GRR is typically a measurement of the variability percentage of the total available engineering tolerance for the part. For any given system, a target maximum GRR may be selected, and various parameters may be chosen or modified to avoid exceeding that GRR.
Metrology system 300 includes a supportive base 303 that may be formed from granite or another material that may aid in isolating sensitive components of the metrology system 300 from ambient vibrations or other disturbances. A stage guide 302 is disposed on the supportive base 303. The stage guide 302 provides a track over which a stage 304 may move as illustrated by movement arrow 305. A fixture 334 is disposed on stage 304 and a workpiece 332 is disposed in the fixture 334. Aspects of the fixture 334 will be further described herein.
In exemplary aspects, the workpiece 332 is a sleeve cone. The sleeve cone is disposed in fixture 334 to provide accessibility to a cavity 338 of the sleeve cone, as illustrated in cross-section in FIGS. 3A-C. An outer surface portion of the workpiece 332 may take any number of shapes, for example, the outer portion may be cylindrical, and need not be conically tapered. Fixture 334 may be adapted to accommodate such variations in workpiece 332.
Additional components of the metrology system 300 include a first sphere 308 coupled by a plunger 328 to a first gauge 314, which outputs information to data acquisition logic 322. First sphere 308 is exemplary and other shapes may be used; for example, a hemisphere may also be used as further described herein. The first sphere 308, plunger 328, and first gauge 314 make up a first measurement assembly. The information outputted by the first gauge 314 may include information describing an amount of extension of the plunger 328. The amount of extension may in turn be used as an indicium of a position of the first sphere 308 in workpiece 332. This indicium of position may be viewed or otherwise interpreted into a height of the first sphere 308 with respect to a reference, in keeping with the conceptual illustration of FIG. 2.
The metrology system 300 also includes a second sphere 310 coupled by a second plunger 326 to a second gauge 312. The second sphere 310, second plunger 326, and second gauge 312 make up a second measurement assembly. The second gauge 312 outputs information that may include information describing an amount of extension of the second plunger 326. The amount of extension may in turn be used as an indicium of a position of the second sphere 310 in workpiece 332. This indicium of position may be viewed or otherwise interpreted into height information of the second plunger 326 with respect to the reference. The amount of extension may be transmitted to the data acquisition logic 322. Other exemplary embodiments may include any amount of preprocessing of extension information. The present invention contemplates metrology using other mechanisms, such as rotatable arms, to extend and retract first sphere 308 and second sphere 310.
The data acquisition logic 322 in turn communicates with a central processing unit 318. The central processing unit 318 interfaces with gauge controller 320 which controls aspects of both the first gauge 314 and the second gauge 312, as discussed below. The central processing unit 318 also interfaces with stage controller 316. Stage controller 316 interfaces with the stage guide 302, thereby controlling movement and positioning of the stage 304. Aspects of the metrology system 300 are further illustrated in FIGS. 3B-C, which show how the spheres 308, 310 are used to measure the sleeve cone angle of the workpiece 332.
FIG. 3A illustrates that stage 304 moves in an exemplary direction indicated by movement arrow 305, and that this movement is initiated by central processing unit 318 controlling the stage through the stage controller 316. FIG. 3B illustrates that stage 304, by direction from the central processing unit 318, positions the workpiece 332 substantially under first sphere 308. In addition, the central processing unit 318 has directed the first gauge 314 to extend first plunger 328 for contacting first sphere 308 to workpiece 332. Based on an amount of extension of the first plunger 328, indicium of a position of the first sphere 308 in the workpiece are determined. Such indicium may include (or may be expressed as) a height of the first sphere 308 with respect to a reference.
FIG. 3C illustrates that first sphere 308 has been retracted and that the stage 304 has moved the workpiece 332 substantially under the second sphere 310. FIG. 3C also illustrates that the second gauge 312 has extended the second plunger 326 so that second sphere 310 contacts the workpiece 332. As described above, these actions may be initiated by central processing unit 318 providing commands or other information to gauge controller 320 and to stage controller 316. Based on an amount of extension of the second plunger 326, indicium of a position of the second sphere 310 in the workpiece are determined. Such indicium may include (or may be expressed as) a height of the second sphere 310 with respect to a reference.
Gauges 312 and 314 may include sensors for determining an amount of extension of the first and second plungers 328, 326. For example, such sensors may include interferometry sensors and associated supporting equipment. Exemplary gauges that may be used include Heidenhain Metro 1287 gauges.
In an embodiment of the invention, the stage controller 316 and the plunger controller 320 interface with the stage 304 and the first and second gauges 314, 312, respectively, at least partially pneumatically. For example, the first and second gauges 314, 312 may each include plunger controls that interface with plunger controller 320 through pneumatic control lines. By applying air pressure through the pneumatic control lines, plunger controller 320 may initiate extension of the first and second plungers 328, 326.
By applying vacuum to those pneumatic control lines, plunger controller 320 may slow extension of, and retract, the first and second plungers 328, 326. Retraction and slowing may also be initiated by spring mechanisms associated with the plunger controls. A rate at which the first and second plungers 328, 326 extend may be controlled to prevent damage to workpiece 332. Timing of slowing extension of the first and second plungers 328, 326 may be controlled to allow rapid extension, and then slowing at a time before contact with workpiece 332. An amount of pressure (vacuum or greater than ambient) and/or volume of gas may be selectable based on the weights of the plungers 328 and 326 and first and second spheres 308, 310.
In a general sense, aspects described in FIGS. 3A-C include fixture 334 dimensioned to hold a workpiece (e.g., workpiece 332) and a first object (e.g., first sphere 308) that is sized to at least fit a portion of the workpiece that is the subject of metrology. The nature of this fit may vary depending on characteristics of the portion of the workpiece subjected to metrology and characteristics of the first object, including size and shape of each.
Also, first plunger 328 is an example of a structural portion for mounting the first object to provide for movement of the first object along a path that results in contact with the workpiece 332. This path, between various metrology uses, need not have precisely the same starting point or ending point, but this path would be expected to lead to contact with the workpiece 332. This path may be predetermined based on the arrangement of the structural portion.
Similarly, plunger controller 320 may be generally viewed as a position controller for the structural portion for mounting the first object. As such, there may be a separate position controller for the first object structural portion and the second object structural portion. Functionality and/or functional portions of each position controller may also be distributed. For example, pneumatic valve(s), motor(s), or other actuators may be included proximate the structural portion, circuitry for controlling that valve(s) may be at a separate location, and computation logic for controlling the circuitry may be at yet another location.
Upon contact, the first object fits with the workpiece at a portion of the workpiece determined by interaction between sizes and shapes of the first object and the workpiece. First gauge 314 is an example of a sensor that can be viewed as sensing a distance traveled by the first object along the path and producing a signal indicative of such distance.
Also, control-related aspects and associated apparatuses, such as stage controller 316, data acquisition logic 322, and plunger controller 320 may be implemented in any of a variety of ways that provide a variety of divisions between mechanical control (e.g., valving, timing, cams, gears, and other devices useful in constructing mechanical apparatuses) and electronic control, between software control running on general purpose processors and application specific hardware implemented in ASICS, FPGAs, or other suitable logic implementations. Aspects relating to second sphere 310, second plunger 326, and second gauge 312 may similarly be generalized.
FIG. 4 illustrates a perspective view of an exemplary implementation of metrology system 300. Base 303 supports stage guide 302. Stage guide 302 includes a first rail 402, a second rail 404, and a top portion 406. A stage 304 interfaces with first rail 402 and second rail 404, which provide guidance to stage 304 as it moves along the stage guide 302. The stage 304 also fits closely to the top portion 406, which can aid in reducing variation of distance between a workpiece 332 disposed in fixture 334 and gauges 314, 312. By reducing variation, the stage can increase accuracy and repeatability because changes in amount of extension of plungers 328, 326 due to such variations would be reduced, and therefore measurement error and variations between measurements would be reduced.
It is known to use an air bearing stage at 304 to achieve a relatively small positioning error. However, air bearings can be prohibitively expensive.
FIG. 5 provides a top view of portions of metrology system 300. Base 303 supports stage guide 302 on which stage 304 moves. Fixture 334 is illustrated as a ring that is placed within a part holding nest 506. The part holding nest 506 defines a surface over which fixture 334 may move under application of force on workpiece 332 (which, as state above, is held by fixture 334) by first sphere 308 or second sphere 310 as each sphere contacts workpiece 332. Workpiece 332 can include a conical cavity to be measured. Although stage 304 may approximately align a bottom of the conical cavity under each of first sphere 308 and second sphere 310, there may be some misalignment, and the first sphere 308 and second sphere 310 may each initially contact workpiece 332 at a point that is not as low in the conical cavity as each sphere may reach. This may occur, for example, when the workpiece is not completely centered under the sphere. This may also occur, for example, when the workpiece is not positioned properly in the fixture (e.g., as low in the cavity of the fixture as possible).
Getting each of first sphere 308 and second sphere 310 as low as possible in the conical cavity of workpiece 332, for example through alignment of the workpiece under each sphere, may increase accuracy of the cone angle measurement. Accuracy of cone angle measurement may additionally or alternatively be increased by proper placement of the workpiece within the fixture.